1. Field of the Invention
In commercial production of panelboard laminates in continuous lengths, more especially those having a polymer foam core sandwiched between facing sheets intimately bonded thereto, difficulty is encountered in obtaining a smooth, planar condition of the upper facing sheet so that it is free of waves, wrinkles and similar imperfections. The present invention is directed to providing a method of, and apparatus for, improving the operation of a conventional nip-roll, free-rising, polymer foam core panelboard processing line to obtain a smooth, planar, surface on the board.
2. Description of the Prior Art
It is well known to produce laminates having a polymer foam core and opposed facing sheets, using continuous, conveyor-like apparatus. Many arrangements are used commercially for this which generally involve advancing a bottom sheet of flexible facing material such as paper, plastic film, metal foil, sheet metal and the like along a conveyor surface, then depositing on that sheet a reactive liquid polymer mix which foams as it ages during forward progress along the conveyor, gradually rising into a blanket or block of rigidified, adherent foam upon completion of the aging process. In the production of double-faced laminate, the upper facing sheet is applied to the surface of the foam mix immediately after lay-down of the mix on the lower sheet, so that the upper facing sheet also becomes bonded to the surface of the foam core as the developing foam produces a rise profile during forward travel along the conveyor.
In order to produce a smooth, unwrinkled, planar face on the laminate, as well as to control the ultimate thickness of the finished product, pressure rolls or platens are conventionally provided between which the foam core/facing sheet laminate is passed as it progresses along the processing line. This is illustrated for example in U.S. Pat. Nos. 3,215,581 and 3,837,771 where the sandwich of opposed facing sheets and polymer foam core is advanced through a curing oven by opposed continuous slat conveyors. These contact the upper and lower surfaces of the laminate and are adjusted toward and away from each other to control the pressure applied to the developing panelboard during the cure process. This controls the resulting thickness of the finished board, both widthwise and lengthwise of the panel. The development of unit pressures due to foam rise over the relatively large areas of the developing board requires slat conveyor apparatus of heavy construction, and accordingly it is expensive to construct as well as cumbersome to operate.
A somewhat similar arrangement is shown in U.S. Pat. No. 3,551,945, but there stationary calibration plates are substituted for the moving pressure slat conveyors. The spacing of the calibration plates is made adjustable, and they are also heated so that passage of the laminate between them helps to cure the foam as well as calibrate the ultimate thickness of the finished panelboard product. The arrangement necessarily involves considerable friction due to sliding of the facing sheets against the plates. U.S. Pat. Nos. 3,574,802 and 4,043,719 show much the same type of arrangement.
In a "free-rising" foam system, attempt is made to meter the amount of reactive liquid foam mix applied to the surface of the lower facing sheet such that the natural, uninhibited rise of the foam will result in the desired final thickness of the core with little or no subsequent application of slat conveyor or platen pressure required. The ultimate objective is to achieve maximum isotropicity of the developed foam cells to get optimum thermal and density properties of the finished product. An equally important objective is that of producing a product which is as nearly flat on both top and bottom faces as possible; i.e. a product of rectangular cross section. However the nature of the foam mix and of the chemical reaction which takes place between the facing sheets introduces conditions which are extremely difficult to control physically, particularly in continuous commercial production. Because of this, consistently uniform density and rectangular cross section are hard to obtain.
In prior bunstock processing systems, which bear some similarity to panelboard processing systems but differ essentially in terms of foam thickness to be dealt with, many devices are employed for ensuring rectangular cross section of the bun in order to minimize scrap or waste when the bun is slit for use in various finished products such as mattresses, cushions, etc. The devices employed include rollers, such as shown in U.S. Pat. No. 3,984,195, to push or press rising, incompletely gelled, foam into desired final bun configuration. Rigid drag panels such as that described in U.S. Pat. No. 4,128,611 have also been used. In the case of bunstock production, the relatively slow rate of conveyor travel (e.g., of the order of 1 foot per minute), plus the substantial height (thickness) of the developed bun (e.g. 2 to 21/2 feet) and the substantial period of time to develope the full rise profile of the foam, make feasible the use of such prior foam shaping devices in those situations since there is room in the apparatus employed to physically place the shaping devices in appropriate position and to adjust their position to a desired point relative to the rise profile of the developing foam bun. That is, in a graphic plot of foam rise vs. time, where the time axis is represented by distance traveled along the conveyor, the rise profile from mix lay-down to fully developed foam height is typically spread out over an interval represented by 15 to 17 feet of conveyor travel.
A different situation exists, however, in producing panelboard laminates. Here the rate of conveyor travel is faster (on average, 10 feet or more per minute). In this case the foam development begins with lay-down of a thin layer of incipiently foamable liquid mix (approximately only one-thirtieth of the desired thickness of the final laminate). This developes to the total thickness (e.g., a maximum of 3 or 4 inches, and usually only one-half to one inch) needed for panelboard production and does so quite rapidly. In panelboard production the rise profile representing development of the foam is accordingly spread over only a few feet along the processing (conveyor) line. This generally makes it impossible or impractical to locate the prior pressure slats, rollers or large drag panels at the proper point or points relative to rise profile, and even more difficult to make adjustment of such placement for needed on-line corrections, because of their cumbersome form.
In panelboard production the amount of smoothing pressure applied also must be of a very low and precisely adjustable amount, due to the low total thickness at any time and relatively high mobility of the foam mix and developing foam. For example, should smoothing pressure be applied at a point in the rise profile where the developing foam is still very fluid (that is, inadequately gelled), the pressure applied simply causes backward squeezing of the liquid mix upstream of the conveyor without necessarily producing the desired planar configuration of the surface. Furthermore, because generally there is horizontal stratification in the foam development, increments of foam mix at the facing sheet interfaces tend to gell before interior increments of the mix. The interior portions thus can still flow and move about easily, while strata near the interfaces is restrained. This adversely affects uniformity of thickness and planarity in the finished laminate.
Another problem is that compression of the developing foam cell structure, if smoothing pressure is applied at the wrong stage of gel development, simply results in stressing the structure of individual cells rather than reorienting cell position in the core matrix. Cell structure stress results in impaired cell isotropicity, reducing desired physical properties of the finished product. Compression for smoothing purposes at the wrong point along the processing line, in other words at the wrong point on the time axis of the rise profile of the foam, may not even manifest itself until after production of the panelboard is completed, as the result of "plastic memory". This can become particularly troublesome under certain ambient conditions to which the foam may be exposed. Warping and surface wrinkling are the usual results here.